Selective Removal of Photocatalytically Active Anatase TiO2 Phase from Mixed-Phase TiO2-ZnO Nanocomposites: Impact on Physicochemical Properties and Photocatalytic Activity

Abstract

TiO2-ZnO nanocomposites were synthesized by varying Ti:Zn molar ratio from 1:0.1 (TZ-1:0.1) to 1:1 (TZ-1:1). With increase in Zn content, from TZ-1:0.1 to TZ-1:0.2, anatase transformed to rutile phase. TZ-1:0.3, which contained a blend of phases, including rutile and anatase TiO2, ZnO, and zinc titanates, exhibited the narrowest bandgap (2.5 ± 0.1 eV), and showed the highest photocatalytic activity. TZ-1:1 was predominated by zinc titanates. All the nanocomposites exhibited narrower bandgaps compared to pure TiO2 nanoparticles, facilitating visible light activity. This study was designed to explore whether a method targeting the removal of a specific crystalline phase (anatase) influenced the properties and photocatalytic activity of the nanocomposite. Selective dissolution not only removed anatase phase, but also led to significant loss of crystallinity, widened the bandgap, and adversely affected photocatalytic performance, in nanocomposites that contained >80% anatase phase (TZ-1:0.1 and TZ-1:0.2). However, in nanocomposites that contained less of anatase phase (TZ-1:0.3 and TZ-1:1), the morphology, bandgap, crystallinity, and the extent of photocatalytic activity at the end of 240 min remained largely unaffected. Photocatalytic activity in TZ-1:0.3 and TZ-1:1 originated from a blend of phases comprising of less photocatalytically active phases, such as rutile TiO2, ZnTiO3, and Zn2TiO4, rather than from the anatase phase. The Ti:Zn molar ratio controlled the phases present in TiO2-ZnO nanocomposites, which, in turn, controlled the physicochemical properties and visible light activity. Thus, in nanocomposites that contained a mix of several phases, the properties and photocatalytic activity were not dependent on anatase phase.